Valley breezes are driven by daytime heating. When the sun shines, the valley floor and the mountain slopes warm up, causing the air near the surface to rise along the slopes. This creates an upslope flow—from the valley floor toward the valley sides or higher terrain—known as the valley breeze. This convective process relies on solar heating, so it occurs most strongly during the day when the sun is out. After sunset, cooling makes the air denser and the downslope flow from the mountains into the valley—called a mountain breeze—becomes more common, so valley breezes are not prevalent at night. Dawn and dusk have transitional winds, but the strongest, most consistent valley breeze happens during daytime.